Collingwood confirmed Brown had received an offer from St Kilda on Sunday afternoon and said it would not match that offer, meaning the restricted free agent is now a Saint.

He was a key player Collingwood's 2010 premiership team, performing crucial shutdown roles on St Kilda skipper Nick Riewoldt in both the Grand Final draw and replay.

However, Brown fell out of favour early this season.

Dropped after round six, he injured his hamstring the following week in the VFL and did not earn a recall to the senior team until round 14.

Despite Jake Carlisle's return from a WADA suspension next season, St Kilda has been actively looking to bolster its key defensive stocks this trade period.

Brown, who turns 28 in December, looks a handy pick-up given he is a strong one-on-one defender and capable of manning elite power forwards such as Josh Kennedy, Taylor Walker, Jeremy Cameron and Jesse Hogan.

Brown and Carlisle will help take the pressure off veteran Sean Dempster inside the Saints' defensive 50 next season, however fellow veteran Sam Fisher is now unlikely to be offered a contract for 2017.

St Kilda chief operating officer Ameet Bains said the experienced defender is a welcome addition to the Saints.

"Nathan is a handy acquisition for the club and will obviously strengthen our defence," he said.

"Bringing Nathan in as a free agent also allows us to fill a key need without compromising our overall trade position, an important consideration given the trades we have completed during this exchange period.

"His ability to hold down a key defensive post makes him an attractive prospect for where our list is currently placed.

"In addition to his experience and leadership on the field, Nathan is a terrific person off the field and we look forward to him joining our ranks."